The present invention relates to an improvement system for thermally conditioning preforms prior to blow molding and to a pallet used therein.
Injection blow molding and reheat blow molding machines typically use pallets to hold preforms during temperature conditioning prior to blowing. U.S. Pat. No. 4,793,960 to Schad, for example, illustrates a blow molding machine that mounts multiple preforms on each pallet. The pallets are entrained and pass between heating elements in ovens along a straight path. Each pallet holds multiple preforms on mandrels, which are rotatable, as the pallet passes between the heaters. Also shown in this patent is the loading of pallets marshalled to accept a complete injection mold shot of parts, the pallets subsequently being entrained to pass sequentially through a series of temperature conditioning ovens.
U.S. Pat. No. 4,824,359 to Poehlsen illustrates an injection blow molding machine employing multiple preform holders that move in a circular path from injection station through three successive thermal condition stations to a blow molding station.
U.S. Pat. No. 4,063,867 to Janniere shows an injection blow molding machine in which preforms are retained on their cores mounted to a core-carrying bar. Six of these bars are successfully loaded into a rotating drum where thermal conditioning of the preforms is carried out while they remain on their cores.
U.S. Pat. No. 4,483,436 to Krishnakumar shows a transport pallet for holding twelve preforms in a neck down orientation within rotatable collets. The pallet has rollers at each end of its upper surface and in pairs along its sides for engaging in track means for guiding the pallet through the machine and through a thermal conditioning system. The collets are rotatable by means of a friction ring mounted to the collet, which engages the side of the track causing rotation of the collet as the pallet passes along the track.
U.S. Pat. No. 4,963,086 to Wiatt shows a reheat blowing machine that shows a belt drive system for rotating multiple collets on a preform carrier during thermal conditioning.
U.S. Reissue Pat. No. 34,177 to Coxhead shows an oven for reheating preforms passing through it mounted on pallet mandrels. The heaters of the oven are individually movable by hand to provide a profiled heating arrangement of the preform length. A detent system is used to record individual heater positions so that these can be reproduced when specific preform styles are repeated.
U.S. Pat. No. 5,834,038 to Ogihara shows an oven for reheating preforms using an oven having vertical heaters aligned with the preform vertical axis matching the pitch between the preforms. The heaters are mounted to hinged plates that can alter the vertical alignment angle of the heaters with respect to the preforms.
U.S. Pat. No. 5,853,775 to Oas et al. relates to a method and apparatus for forming stretch blow molded containers in which parisons are heated non-uniformly by rotating at a non-uniform rate in a heating station. A sensor determines the angular orientation of the parisons emerging from the heating station. Each parison is angularly reoriented at a repositioning station prior to introduction into a stretch blow molding station having non-round interior surfaces, so that the temperature profile of each parison corresponds with differential expansion required to form a non-round container.
These patents however suffer from a number of deficiencies. For example, they do not teach the concept of relative movement between the preform pallet and the thermal conditioning device for providing clearance therebetween. Nor do they teach any mechanism for altering the heating profile while the blow molding machine is operating. When the pallets are entrained in a circular path as they pass through the temperature conditioning means, there is a need to optimize their position with respect to the heating elements to ensure all the preforms carried on the pallet are thermally conditioned equally.